The broad, long term goal of this project is to use vertical movement of the walking surface as a means to study the mechanical and sensorimotor contributions to gait entrainment in healthy, walking humans. This project is based upon the previous work of investigators who have studied the tendency for humans to synchronize movement with various external cues, and have begun to apply these ideas to the rehabilitation of gait in individuals with mild to moderate impairment. While the use of synchronization as a therapeutic exercise has demonstrated significant potential for enhancing locomotor function in several patient populations, much remains unclear regarding the neuro-mechanical properties that underlie this behavior. Further, recent research suggests that current forms of synchronization therapy used for restoring gait function may not be optimal. Additional research is needed to understand the factors that contribute to this behavior, as well as the most effective techniques by which this type of intervention might be applied. This project is specifically designed to investigate entrainment using a motorized platform and overhead support harness. Based upon the wobbly bridge phenomenon whereby humans synchronize to movement of the walking surface, this approach will provide an effective mechanism for studying this type of gait entrainment. This approach is attractive because it will use mechanical input for synchronization rather than visual or auditory cues. Previous study suggests that mechanical input is more likely to result in entrainment of gait and will therefore more reliably induce the desired behavior. In addition, this approach provides a means to drive entrainment with a highly controllable forcing function. The ability to control the external signal will be very important for systematic study of entrainment behavior. Finally, previous investigators have utilized horizontal movement as an external perturbation to stepping, but vertical movement will afford a fundamentally different and novel analysis. There are three objectives to this project: 1) to experimentally establish the relationship between phase locking, frequency of the forcing function (i.e. detuning factor), and coupling strength in the form of an Arnold tongue diagram for human gait, 2) to establish phase resetting curves in response to both discrete and periodic perturbation through vertical movement of the walking surface, and 3) to elucidate the role of vision in gait entrainment to a moving walkway by manipulating both walking surface and visual flow. Taken together, achievement of these objectives will provide substantial insight to gait entrainment and overall control of locomotion in humans. Due to the novelty of this type of perturbation, these data will also have application to understanding the response to external perturbation and will improve gait models and measures for predicting fall. Finally, movement of the walking surface may eventually be useful itself as an effective means to apply gait entrainment therapy.